1. Field of the Invention
The present invention relates to a polypropylene fiber having both adequate strength and high flame retardancy and generating no toxic gas at the time of burning, a production method thereof and a flame retarding polypropylene film.
2. Description of the Related Art
As polypropylene fiber products having flame retardancy, those obtained by bestowing a halogen-based retardant to fiber products composed of, for example, a polypropylene fiber by post processing have been conventionally general. Or, for example, a flame retarding fiber obtained by kneading a flame retardant in a fiber which is obtained by preliminarily adding a flame retardant to a polypropylene resin to be melt spun has been also known.
It has been recently required to use a non-halogen-based retardant which does not also generate a toxic gas even at combustion from an environmental viewpoint. As a product in which a non halogen-based flame retardant is used, for example, a polyolefin-based flame retarding adhesive tape disclosed in Japanese Patent Laid-Open Publication No. 9-310048 is disclosed. The tape is provided with an adhesive on the tape substrate, mainly composed of a polyolefin-based resin and a flame retardant. The tape substrate is compounded with an inorganic flame retardant as a flame retardant, a hindered amine-based compound as a weather resistant agent, and a hydrotalcite compound as a weather resistant aid for the polyolefin-based resin, kneaded, dispersed and made in a sheet. The inorganic flame retardant includes magnesium hydroxide, aluminum hydroxide, stannic oxide, antimony oxide, red phosphorous and the like.
Further, a cross-linking flame retarding composition having good heat resistance which is used for molded articles such as a film, a sheet, a pipe, a container, an electric wire, a cable and the like is disclosed in Japanese Patent Laid-Open Publication No. 7-102128. The composition is composed of a predetermined resin material including a polyolefin-based resin, a flame retardant and red phosphorous at a predetermined proportion. As the flame retardant, addition-type flame retardants such as a halogen-based retardant, a phosphorous-based flame retardant, an inorganic flame retardant and the like may be used. Further, it is suggested that higher flame retardancy can be further bestowed by compounding red phosphorous.
For example, a flame retarded polypropylene compound suitable for producing a stampable sheet excellent in flame retardancy is disclosed in Japanese Patent Laid-Open Publication No. 11-60837. A specific amount of a metal hydrate as a flame retardant for a polypropylene is contained in the flame retarding polypropylene compound. Thus, it is suggested that mesh choking caused by the melt adhesion of a pulverized article (compound) according to pulverization at room temperature can be also suppressed by compounding the specific amount of the metal hydrate.
However, the product obtaining flame retardancy using these flame retardants has a high concentration of compounding a flame retardant. Specifically, given that the resin component is 100 by weight, the flame retardant is 30 to 200 by weight in the polyolefin-based flame retarding adhesive tape disclosed in Japanese Patent Laid-Open Publication No. 9-310048, 5 to 200 by weight in the cross-linking flame retarding composition of Japanese Patent Laid-Open Publication No. 7-102128, and 50 to 200 by weight in the flame retarded polypropylene compound of Japanese Patent laid-Open Publication No. 11-60837.
Any one of the resin materials disclosed in these Publications is a sheet and various kinds of molded articles, and a use as a fiber is not intended. Accordingly, there is no problem in strength even if an extremely high concentration of a flame retardant as described above is added, but when such resin materials are used for fiber uses or thin film, only a fiber or film physically having low strength is obtained. And there may be problems in stability at the time of spinning and film forming.
Further, a light resistant stabilizer is usually added in the polypropylene fiber in order to maintain fiber physical properties, but the light resistant stabilizer is deactivated by a flame retardant, and there is a great problem that the lowering of strength in a lapse of time is remarkable.
On the other hand, it is disclosed in International Patent Publication (Unexamined) No. WO99/00450 (PCT/US98/13469) that in particular, an NOR type compound among hindered amine-based compounds known as a light resistant stabilizer reveals flame retardancy. Further, it is disclosed in the same Publication that the NOR type hindered amine-based compound can be added in various resin materials and 0.25 to 1.0% by weight of the NOR type hindered amine-based compound is added as the flame retardant of a polypropylene fiber in its Example. The polypropylene fiber containing 1.0% by weight of only the NOR type hindered amine-based compound shows the same level of flame retardancy as a polypropylene fiber containing 3% of a bromic acid-based flame retardant which is widely known as the flame retardant used for a conventional fiber.
Further, the content of the various NOR type hindered amine-based compounds is changed to be added for various fibers containing a propylene fiber in the same Publication, and flame retardancy thereof is studied. However, properties other than initial flame retardancy, for example, the durabilities of the fiber strength and the flame retardancy are not studied in the International Patent Publication (Unexamined), therefore there is no description thereof at all. Further, it is disclosed in the International Patent Publication (Unexamined) that other flame retardants such as a phosphorous-based compound, a halogen-based compound, and the like are used with the NOR type hindered amine-based compounds in combination and the whole amount is 0.5 to 20% by weight. However, a specific study in case of containing a flame retardant in the polypropylene fiber in combination is not carried out.
Further, since the NOR type hindered amine-based compound which functions as the flame retardant has also a function as a light stabilizer, the same NOR type hindered amine-based compound lowers both functions in a lapse of time, and cannot keep a fixed flame retardancy over a long period.